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Mid-Range High-Pass Crossover

03-13-2018, 07:41 PM

Would it be correct to say that a dome mid-range (with back cup) that begins to roll off acoustically at 800 Hz at 12 dB/octave indicative of a sealed system should be electrically crossed over higher than the natural acoustic roll off to both limit distortion associated with high excursion and suspension non-linearities, but also to limit low frequency energy turning into voice coil temperature rise? If all we care about in a crossover is the final acoustic slope regardless of how many components it takes to achieve that, are these the only good reasons not to run the dome mid-range full-range and let it cross over naturally to the woofer at 800 Hz (assuming we are targeting a 2nd order crossover at 800 Hz)?

I understand output is a factor, and louder playback will require larger excursions; so is fear of breaching xmax a factor? Would it just sound awful, lol? Should I assume that the natural acoustic roll-off at 800 Hz results from the driver exceeding its linear range and so we should cross it higher to keep it within its linear range?

Lets substitute that dome mid-range for a cone shaped driver that has output potential far below the desired crossover point. Let's put that driver in a high Q, tiny box. If the driver is crossed an octave or two (or even higher) above the fc of the under-damped system, do we care that the system is under-damped apart from any potential increase in distortion resulting from the poorly tuned system?

As a final hypothetical, what if we put that cone shaped driver in a perfectly aligned box that achieved our desired crossover slope to the sub acoustically (lets say, 100 Hz). Would we need to electrically cross the mid-range in any way? What if we got that mid-range to roll off closer to 200 - 300 Hz?

Without a crossover in place, the driver will still try to reproduce whatever signal it is given. The natural roll off is not enough to protect the driver against the demands of bass input. This is regardless if the driver is a dome with its own sealed chamber or a cone placed into a chamber. In the case of the cone driver, using an undersized, high q chamber may result in other problems as well; midrange energy that is reflecting back into the cone resulting in distortion.

It is possible to use the enclosure as part of the roll off. An under sized enclosure with heavy fill can work. The smaller enclosure will aid in limiting the xmax, but that also comes at the expense of sensitivity. The 12db/octave roll off simply may not be enough under load. And really simply adding in a capacitor (resulting in a 3rd order acoustic slope) is simple and cheap to do. A higher crossover point is possible, but can still have the issues listed above. Better to use a few components that protect the drivers and result in a smooth transition between them.